1. Field of the Invention
This invention is generally related to laser processing of materials and more specifically to an improved method of welding or sealing together the edges of juxtaposed, spaced apart glass sheets in a vacuum.
2. Description of the Prior Art
In our U.S. Pat. No. 4,683,154, which is incorporated herein by reference, we described a laser sealed vacuum insulation window. The window comprises two juxtaposed sheets of glass held spaced apart in relation to each other by a plurality of spherical glass beads, and the edges of the juxtaposed glass sheets are glass-welded together to provide a sealed, evacuated space between the juxtaposed glass sheets that is completely glass-sealed from the exterior. The edges of the juxtaposed glass sheets are welded together in a vacuum chamber by a laser beam that is focused and steered around the edges of the glass sheets. The laser used to melt and weld the edges together had to be of a wavelength that is absorbed by the glass and of sufficient power to raise the temperature of the glass edges to the melting temperature of the glass, which is about 1,200.degree. C. to 1,300.degree. C. for borosilicate glass.
Unfortunately, the laser welding of the edges of the juxtaposed glass sheets to seal the vacuum space has been hindered by several technical problems that occur when the glass is heated above its annealing temperature in the vacuum chamber. First, dissolved gases in the glass tend to nucleate bubbles in the weld line, which become defects that can produce flaws and failures of the vacuum tight seal. Second, the molten glass at the weld point vaporizes and contaminates the mirror in the vacuum chamber that is used to steer the laser beam around the edges of the glass sheets being welded, which decreases specularity of the mirror and diffuses the laser beam, thus causing a loss of power and frustrating the welding process, and, if allowed to continue, could result in excessive absorption of the laser beam and consequent heat damage to the mirror.